A scroll expander includes a stationary scroll and an orbiting scroll that are meshed with each other. An end plate and a scroll lap mounted upright on the end plate are provided on each of the stationary scroll and the orbiting scroll. In the scroll expander, an expansion chamber is formed between the end plate and lap of the stationary scroll and the end plate and lap of the orbiting scroll. The orbiting scroll moves in a circular orbit while being restricted in self rotation by a rotation-restricting mechanism. When the orbiting scroll thus revolves, the expansion chamber moves while changing its volumetric capacity, thereby carrying out suction, expansion and discharge of a fluid.
The expansion chamber is formed on each of the inner wall side and the outer wall side of the lap of the orbiting scroll. The expansion ratio of the expansion chamber on the inner wall side of the lap (hereinafter referred to as an inner wall side expansion chamber) and that of the expansion chamber on the outer wall side of the lap (hereinafter referred to as an outer wall side expansion chamber) are determined respectively by the shapes of the laps of the orbiting scroll and the stationary scroll. For example, as disclosed in JP 08(1996)-28461 A and JP 2002-364563 A, in a conventional scroll expander, both laps provided on a stationary scroll and an orbiting scroll are formed in such shapes that the expansion ratio of the inner wall side expansion chamber and that of the outer wall side expansion chamber are equal to each other.
The expansion chambers of a conventional scroll expander are described below with reference to FIG. 15. This scroll expander includes a stationary scroll having a lap 502 and an orbiting scroll having a lap 501. An inner wall side expansion chamber 503a is formed on the side of a lap inner wall 501a of the orbiting scroll, and an outer wall side expansion chamber 503b is formed on the side of a lap outer wall 501b thereof.
In the case of a refrigeration cycle apparatus provided with this scroll expander, a fluid to be expanded is a refrigerant. The refrigerant is drawn through a suction port 507 provided in the center of the scrolls. The drawn-in refrigerant expands and moves toward the outer peripheral portions of the respective scrolls along with a change in the volumetric capacities of the expansion chambers 503a and 503b, and is discharged from a discharge port 506.
FIG. 15 illustrates the moment when the radially innermost expansion chambers 503a and 503b shift from the suction process to the expansion process. In other words, FIG. 15 shows the moment when, in the center of the scrolls, the lap inner wall 501a of the orbiting scroll and the lap outer wall 502b of the stationary scroll come in contact with each other, and the lap outer wall 501b of the orbiting scroll and the lap inner wall 502a of the stationary scroll come in contact with each other, that is, the moment when contact surfaces 504 and 505 are created. As is apparent from FIG. 15, the inner wall side expansion chamber 503a and the outer wall side expansion chamber 503b are closed at the same time.
As the expansion process proceeds, the contact surfaces 504 and 505 move toward the outer circumference of the scrolls following the shapes of the laps, and eventually disappear at the same time in the outermost peripheral portion of the scrolls. That is, the inner wall side expansion chamber 503a and the outer wall side expansion chamber 503b are opened at the same time. In this scroll expander, the involute lap on the lap inner wall 502a of the stationary scroll is terminated at a midway position 502c, so that a position where the contact surface 504 between the lap inner wall 501a of the orbiting scroll and the lap outer wall 502b of the stationary scroll disappears is displaced by 180 degrees from a position where the contact surface 505 between the lap outer wall 501b of the orbiting scroll and the lap inner wall 502a of the stationary scroll disappears. Thereby, the inner wall side expansion chamber 503a and the outer wall side expansion chamber 503b are opened at the same time.
As described above, in the conventional scroll expander, the inner wall side expansion chamber 503a and the outer wall side expansion chamber 503b start closing at the same time and start opening at the same time, that is, the expansion processes in the respective expansion chambers 503a and 503b start at the same time and finish at the same time. As a result, the expansion ratios of these two chambers 503a and 503b are equal to each other.
However, since the expansion ratios of both the expansion chambers 503a and 503b of the above-mentioned scroll expander are fixed all the time, it cannot necessarily perform an efficient expansion operation constantly in such an application as a refrigeration cycle apparatus in which the preferred expansion ratio varies according to the operation conditions.
To be more specific, when a scroll expander is used for a refrigeration cycle apparatus, for example, the high pressure and the low pressure of the refrigeration cycle vary as the operation conditions of the refrigeration cycle apparatus change. The suction pressure and the discharge pressure of the expander also vary accordingly. However, since the expansion ratios of the expansion chambers are preset to a fixed design ratio, as described above, over-expansion or under-expansion of a refrigerant may occur in the expander depending on the values of the suction pressure and the discharge pressure.
FIGS. 16A to 16C show pressure-volume diagrams in an expansion process. FIG. 16A shows a case where the expansion ratio of an expansion chamber coincides with the high pressure/low pressure condition of a refrigeration cycle apparatus. In other words, it shows a case where the expansion ratio of the expansion chamber is equal to the pressure ratio between the high pressure and the low pressure of the refrigeration cycle apparatus. In this case, no loss occurs in the expansion process.
On the other hand, FIG. 16B shows a case of an operation condition where the high pressure is higher and the low pressure is lower respectively than the high pressure/low pressure condition of the refrigeration cycle apparatus of FIG. 16A (Ph2>Ph1 and Pl2<Pl1). This operation condition occurs in a case where heat is radiated when a temperature outside a radiator is higher and heat is received when a temperature outside an evaporator is lower respectively than the temperatures of the operation condition of FIG. 16A. The suction volume and the discharge volume of the expansion chamber are designed so that the refrigerant expands just enough under the high pressure/low pressure condition of Ph1/Pl1. Therefore, assuming that the pressure Ph2 of the refrigerant to be drawn into the expansion chamber is greater than Ph1, the refrigerant to be discharged from the expansion chamber cannot expand to reach the low pressure Pl2 of the refrigeration cycle apparatus, thereby being discharged at a higher pressure than Pl2. As a result, under-expansion occurs under the operation condition of FIG. 16B, thereby causing a loss as shown in a diagonally shaded area in FIG. 16B.
FIG. 16C shows a case of an operation condition where the high pressure is lower and the low pressure is higher respectively than the high pressure/low pressure condition of the refrigeration cycle apparatus of FIG. 16A (Ph3<Ph1 and Pl3>Pl1). This operation condition occurs in such a case where heat is radiated when a temperature outside a radiator is lower and heat is received when a temperature outside an evaporator is higher than the temperatures of the operation condition of FIG. 16A. As described above, the suction volume and the discharge volume of the expansion chamber are designed so that the refrigerant expands just enough under the high pressure/low pressure condition of Ph1/Pl1. Therefore, assuming that the pressure Ph3 of the refrigerant to be drawn into the expansion chamber is smaller than Ph1, the refrigerant to be discharged from the expansion chamber expands to exceed the low pressure Pl3 of the refrigeration cycle apparatus, thereby being discharged at a lower pressure than Pl3. As a result, over-expansion occurs under the operation condition of FIG. 16C, thereby causing a loss as shown in a diagonally shaded area in FIG. 16C.
As described above, a refrigeration cycle apparatus or the like including a conventional scroll expander can perform a highly efficient operation as long as the high pressure/low pressure of the refrigeration cycle apparatus coincides with the design expansion ratio of the scroll expander. However, on the other hand, even a small change in operation conditions easily increases a loss caused by under-expansion or over-expansion. Therefore, the expander deteriorates in mechanical power recovery performance, which results in difficulty in sufficiently enhancing the capability of the refrigeration cycle apparatus.